1. Field of the Invention
The present invention relates to an optical scanner installed in an image forming apparatus in an electrophotographic system, and to an image forming apparatus such as a laser printer, a laser plotter, a digital copying machine, a plain paper facsimile, or a complex machine thereof, using the optical scanner.
2. Description of the Related Art
Recently, in an image forming apparatus in an electrophotographic system, which is known as a laser printer, the demands for a laser plotter, a digital copying machine, a plain paper facsimile, or a complex machine thereof, full-color, high quality, high speed, small size, and low cost have increased. In order to achieve such high speed in image forming by the image forming apparatus, image forming apparatuses in a tandem system, in which a plurality of photoconductors as image carriers, normally four photoconductors, are provided have come into wide use. In the tandem system image forming apparatus, for example, four photoconductors are disposed in parallel along a transfer belt or an intermediate transfer belt configured to convey a recording medium. After each photoconductor is charged by a corresponding charging unit, a latent image is formed on each photoconductor by a writing unit, and then the latent image formed on each photoconductor is developed with a developer corresponding to a different color (for example, yellow, magenta, cyan, or black) by a corresponding development unit to be visualized as a toner image. The toner image corresponding to each color is transferred in a superimposed state on a recording medium fed by the transfer belt or the intermediate transfer belt to form a color image.
As the writing unit used in such a tandem system image forming apparatus, an optical scanner is known in which surfaces to be scanned are disposed at opposite sides of an optical deflector commonly-used in a plurality of optical systems, and light beams are emitted from both sides of the optical deflector to scan the surfaces is known. According to the above configuration, exposures of four colors, for example, yellow, magenta, cyan, black can be performed substantially at the same time so that high-speed of the image forming can be achieved.
In the case of the optical scanner in which the surfaces to be scanned are disposed at the opposite sides of the commonly-used optical deflector, when a synchronization detection is performed before writing, the light beams to scan the surfaces to be scanned are received by a light-receiving element prior to the start of writing in order to perform writing start synchronization. If the optical deflector is commonly-used by the plurality of scanning optical systems, one surface to be scanned is scanned in a reverse direction of a direction where the opposite surface to be scanned is scanned, that is, a writing start position in each scanning optical system is disposed at a reverse side with respect to a plane which passes a rotational center of the optical deflector and perpendicular to the surfaces to be scanned.
Accordingly, the light-receiving elements of the scanning optical systems disposed at the opposite sides of the optical deflector are disposed at opposite sides in the optical scanner. In the above-described configuration, four light paths are confusingly disposed in the optical scanner such that their arrangement is complicated and it is difficult to achieve a small size apparatus without interference. In particular, light paths for the synchronization detection need to be provided out of the effective scanning area in the main scanning direction. The number of optical elements required to perform the synchronization detection is preferably as few as possible to achieve a small size and low cost device. However, if the number of optical elements is reduced, it is necessary to prevent degradation of image quality, which is caused by degradation of optical properties. In the above-described scanning optical systems, it is necessary to ensure wide areas to perform the synchronization detection in relation to effective scanning widths. For example, if in both of the two oppositely disposed scanning optical systems, the synchronization detection is performed before scanning, as shown in FIG. 2, right-hand and left-hand scanning optical systems are mutually deviated from each other so that a wide exposure width is required. Therefore, a large size optical deflector is required, making it difficult to achieve high speed and long-term performance and there is a problem in that noise is generated due to rotation of the optical deflector.
To solve the above problems, in conventional technology, the following improvements have been made.
Japanese Patent Application Publication Number 2004-85969 discloses an optical scanner having a plurality of scanning optical systems to scan different surfaces to be scanned. In this case, in the pair of scanning optical systems, reference reflection positions, each of which is defined as a position of the reflection surface of the optical deflector, where the light beam reflected on the reflection surface of the optical deflector is perpendicular to the surface to be scanned, are asymmetric with respect to a reference plane which includes a rotational axis of the optical deflector and is parallel to both surfaces to be scanned. The scan lenses of the pair of scanning optical systems are symmetric with respect to a line which passes a rotational center of the optical deflector and which is parallel to the surfaces to be scanned. That is, both pairs of scanning optical systems are configured to perform the synchronization detection before scanning and the reference reflection positions of the pair of scanning optical systems are mutually asymmetric with respect to the reference plane. However, the reflection positions of the scanning optical systems are different, and therefore distances from the rotational center to the reflection surfaces are different so that the positional deviation of a beam spot in the main scanning direction due to the effect of sags and degradation of a curvature of an image in the main and sub scanning directions occur causing degradation of the output image.
Japanese Patent Application Publication Number H04-313776 discloses a prior art in which the synchronization detection is performed by a light-receiving element to determine a writing start position. That is, based on positional information on one writing start position, the other three writing start positions are determined. In this method, although only one light-receiving element is provided so that layout freedom increases, variations in wavelength of the different light sources are not considered and time intervals become large so that accuracy is degraded and variations in writing start positions cause color deviations in an output image.
In the related arts described above, in the optical scanner in which scanning is performed at opposite sides of the optical deflector, it is not possible to obtain an optical scanner which performs high-quality writing while providing layout freedom of the light-receiving element to perform synchronization detection.